A hybrid helicopter relates to an advanced concept for a vertical takeoff and landing (VTOL) aircraft.
This advanced-concept hybrid helicopter, as described in document FR 2 916 418, associates at reasonable cost the effectiveness in vertical flight of a conventional helicopter with the high-speed travel performance made possible by using propulsive propellers and modern turbine engines.
Thus, a hybrid helicopter is neither a helicopter nor an autogyro nor a gyrodyne. Likewise, a hybrid helicopter is neither a compound rotorcraft nor a convertible rotorcraft.
A hybrid helicopter has a fuselage and a main lift rotor for driving blades in rotation under drive from at least one turbine engine.
Furthermore, a hybrid helicopter has a wing made up of two half-wings, with the left and right propulsive propellers being placed on either side of the fuselage, on the half-wings.
In addition, a hybrid helicopter is fitted with an integrated drive system including a mechanical transmission interconnecting the turbine engine(s), the main rotor, and the left and right propellers.
For convenience in the text below, it is assumed that the left of an aircraft is to the left of a person in said aircraft facing towards the front of the aircraft, i.e. with the back towards the rear of the aircraft. As a result, it is considered that the right of an aircraft is on the right of a person in said aircraft facing towards the front of the aircraft and with the back directed towards the rear of the aircraft.
The terms “left” and “right” as used below are thus respectively equivalent to the terms “port” and “starboard” as used at sea.
With such a configuration, the speeds of rotation of the outlet(s) from the turbine engine(s), of the left and right propellers, of the main rotor, and of the interconnecting mechanical transmission are mutually proportional, with the proportionality ratios being constant regardless of the flight configuration of the hybrid helicopter and under normal conditions of operation of the integrated drive system.
Consequently, and advantageously, the main rotor is continuously driven in rotation by the turbine engine(s) and it always develops lift regardless of the configuration of the hybrid helicopter, both in forward flight and when hovering.
Thus, during stages of flight in a normal configuration, i.e. excluding the flight configuration involving autorotation either for the purpose of engine breakdown training or as a result of a genuine engine breakdown, said at least one turbine engine is always mechanically connected to the rotor. The rotor is thus always driven in rotation by said at least one turbine engine regardless of flight stage when in a normal configuration, with it being understood that an autorotation flight configuration does not form such a normal configuration flight stage, for example.
More precisely, the main rotor serves to provide the hybrid helicopter with all of its lift during stages of take-off, landing, and hovering, and to provide some of its lift in cruising flight, with the wing then contributing part of the lift supporting said hybrid helicopter.
Thus, the main rotor provides the major fraction of the lift for the hybrid helicopter in cruising flight, possibly together with a small contribution to the propulsive or traction forces, and always with minimum drag.
Like a helicopter, the pilot has first and second control members, a collective pitch lever and a cyclic pitch stick, for example, for the purpose of controlling respectively the collective pitch and the cyclic pitch of the blades of the main rotor.
Furthermore, by modifying the pitches of the blades of the left and right propellers of the hybrid helicopter collectively and by the same amount, it is also possible to control the thrust generated by the left and right propellers.
The pilot thus has at least one third thrust control member such as a control lever, suitable for modifying the pitches of the blades of the right and left propellers collectively and by the same amount.
In contrast, the antitorque and steering functions are achieved by making use of differential thrust exerted by the left and right propellers, e.g. by the pilot making use of a differential pitch control member of the rudder-bar type.
Consequently, the third control member serves to define the mean pitch of the blades of the left and right propellers, where the mean pitch corresponds to half the sum of the first and second pitches of the blades of the left and right propellers.
In contrast, the rudder bar serves to cause the pitches of the left and right propellers to depart from the mean pitch in differential manner, with the pitch of the blades of one propeller being increased by a differential pitch value while the pitch of the blades of the other propeller is decreased by said value.
In order to provide the antitorque function on a hybrid helicopter, it is thus appropriate to obtain differential thrust, with the value and/or the direction of the thrust exerted by the left propeller necessarily being different from the value and/or the direction of the thrust exerted by the right propeller. It can thus be concluded that the first pitch of the blades of the left propeller is necessarily different from the second pitch of the blades of the right propeller in order to stabilize the hybrid helicopter in yaw.
Nevertheless, the maximum efficiency of a propeller can be obtained only for a given value for the pitch of the blades of said propeller for any given advance factor λ, where the advance factor λ is given by:λ=TAS/(Ω×R)where TAS is the true air speed of the rotorcraft, Ω is the speed of rotation of the propellers, and R is the blade radius of the propellers.
Since the left and right propellers are identical, a pitch difference between said first and second pitches necessarily implies that at least one of the propellers is not operating in its best efficiency range.
This difference also gives rise to a difference between the first torque generated by the left propeller and the second torque generated by the right propeller that can, under extreme circumstances, become problematic. If one propeller is close to its maximum admissible torque, then the pilot's margin for maneuvering in yaw with the propeller that is closest to its limit consequently becomes limited.
It is possible to envisage fitting the hybrid helicopter with aerodynamic surfaces capable of generating transverse antitorque lift, stationary fins extending substantially parallel to the plane of symmetry of the fuselage and arranges at the rear end of the fuselage.
These aerodynamic surfaces may also be fitted with rudder flaps, and electric actuators that control the flaps so as to vary their angles of deflection relative to the stationary fins to which the flaps are fastened, for the purpose of stabilizing the hybrid helicopter.
Document U.S. Pat. No. 4,935,682 describes a control device for preventing an airplane that has thrusters disposed on either side of its fuselage from beginning to perform movement in yaw in the event of the thrust generated by one of the thrusters becoming lower.
Thus, according to that Document U.S. Pat. No. 4,935,682, the control device makes use of a differential thrust signal to control the position of a steering rudder.
Document EP 0 742 141 seeks to obtain the same result by controlling the position of a rudder as a function of airplane accelerations.
Documents FR 2 689 854 and FR 2 769 285 present a helicopter provided with a steering aerodynamic surface that exerts transverse lift, said aerodynamic surface including a rudder flap of adjustable deflection angle relative to the aerodynamic surface.
Thus, according to document FR 2 689 854, the deflection angle of the flap is controlled automatically as a function of the collective pitch of the main lift rotor of the helicopter and as a function of the speed of advance of said helicopter.
Finally, document U.S. Pat. No. 6,478,262 presents an aircraft having a steering rudder that operates in conjunction with the collective pitch control to perform the yaw-control function.
Document EP 0 867 364 presents a helicopter having vertical fins, the angle of attack of the fins varying in order to control the helicopter in yaw. The effectiveness of the tail rotor is preponderant at low speed in countering the torque exerted by the main rotor on the airframe, whereas the effectiveness of the fins is preponderant at high speed.
Thus, the state of the art does not provide precise teaching for optimizing the operation of left and right propellers of a rotorcraft having a main lift rotor driven by a power plant during at least one stage of flight. The state of the art does not specify how the blades of the left and right propellers can be enabled to have pitches that are close or even equal to the given value that gives rise to maximum efficiency for each propeller.
That state of the art does no more than present means for stabilizing an aircraft and controlling it in yaw. Consequently, the above-mentioned documents do not make it possible to solve the problem that is posed.